This invention relates to airspeed transducers, and more particularly, to an airspeed transducer having a pitot tube that inherently provides a non-linear (squared) pressure indication of airspeed, in combination with a capacitive pressure sensor and an electrostatic force-feedback system to provide a resulting linear voltage indication of airspeed.
Airspeed is typically measured on an aircraft by a pitot tube sensor that is usually located at the nose of the aircraft or on other forward surfaces. On rotorcraft, the pitot tube may be mounted near the nose of the craft or stationary at the center of the main rotor shaft. Regardless of its location, the pitot tube is generally required to operate over a wide dynamic range of airspeeds; for example, from 0 to 200 knots. If the airspeed measuring system is designed for a 200-knot maximum airspeed, the dynamic range of prior art pitot tube pressure sensing systems is practically limited to speeds above 30 knots. This is usually not a problem for fixed-wing aircraft, because at airspeeds less than 30 knots, they are unable to fly due to loss of lift.
However, the inability to accurately measure airspeeds under 30 knots is a problem for rotorcraft. This is because helicopters perform many useful in-flight functions at less than 30 knots. Yet, helicopters usually also require airspeed indication out to 200 knots. Thus they have larger airspeed dynamic range sensitivity requirements compared to fixed-wing aircraft.
The use of a pitot tube to measure airspeed is further complicated by the fact that the pressure output of the pitot tube is a squared function of airspeed. That is, the pitot tube typically comprises two pneumatic lines, one for dynamic airflow and one for static airflow. The pitot tube output is taken as the difference between the pressure of the dynamic airflow and the pressure of the static airflow. This pressure differential is inherently proportional to the square of the speed of the airflow.
The task is then to linearize this squared airspeed using various non-linear means. It is known in the prior art to use non-linear mechanical means such as meter scales and movements. It is also known to use conventional diaphragm-type pressure transducers, which provide an output that is still proportional to airspeed squared, and then follow that with non-linear electronic circuits. For example, it is known to use a capacitive pressure sensor connected to the pitot tube output. A sensor diaphragm deflects according to the static and dynamic airflow pressures, thereby changing the capacitance of the sensor. This changing capacitance is typically processed electronically in an open-loop, non-linear fashion to indicate airspeed. In this case, the overall system linearity is no better than the combination of the linearity of the diaphragm deflection with pressure and the linearity of the signal processing electronics.
Accordingly, it is a primary object of the present invention to provide an airspeed sensor that has improved performance over prior-art designs at low airspeeds.
It is a general object of the present invention to provide a pressure transducer that yields a linear output for airspeed when the input is the pressure output from a pitot tube.
It is another object of the present invention to provide a closed-loop, electrostatic force-feedback system for a capacitive pressure sensor that linearizes a non-linear indication of airspeed from a pitot tube.
It is yet another object of the present invention to provide an airspeed transducer operable over a wide range of airspeeds.
It is still another object of the present invention to provide an airspeed sensor utilizing a differential capacitive pressure sensor that acts as both a displacement sensor for measuring the force on a variable, movable diaphragm of the sensor due to the airspeed, and as a counterbalance to this force by application of an electrostatic force to a fixed plate of the sensor to restore the diaphragm to an undeflected position.
It is yet another object of the present invention to provide an airspeed transducer that has improved sensitivity to relatively low airspeeds.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.